The present invention relates to safety devices for lifting mechanisms, and more particularly to emergency stop mechanisms for lifting mechanisms using winch-powered cables or lines. Such lifting devices are well known and in common use, for example in elevators, dumbwaiters and in scissors mechanisms for raising industrial or agricultural equipment.
Such lifting mechanisms often consist of a winch operated cable or line, a pulley system and a trolley or traveling member. The trolley may be a sliding member or, as in the case of an elevator, a vertically translating platform or the payload itself. Frequently, the failure of any component of the lifting mechanisms will result in a loss of tension in the cable or line and, therefore, in an absence of lifting force to counteract the force of gravity. The outcome of such a failure may be undesired rapid downward movement of the payload.
Strengthening some of the individual components of the lifting mechanism may increase the reliable operation of the system and greatly reduce the chance of mechanical failure, but there are practical limitations of cost and size to such strengthening. Furthermore, the lifting mechanism will be only as reliable as its weakest component and the reliable operation of the lifting mechanism can be significantly affected only if all the components are strengthened.
Providing a complete second and redundant lifting mechanism is another alternative, but an extremely costly one. Furthermore, repeated use of the parallel lifting mechanism places the same loads on the backup mechanism as on the primary mechanism, thus evenly wearing components of both mechanisms. When one of such mechanisms fails from age, misuse, or faulty maintenance, the other mechanism will probably also be in a weakened state. The increased load suddenly placed on the surviving mechanism could cause it to fail also, thus releasing the payload.
Many attempts have been made at designing safety devices to prevent the free fall of the payload of lifting mechanisms, some particularly well suited to mechanisms lifting extremely heavy or extremely fragile payloads and others designed for a lifting mechanism moving the payload up and down along a stationary vertical shaft. U.S. Pat. No. 1,072,362, for example, disclosed an example of a safety mechanism for mine cages having guide ropes wherein a clutch normally restrained by the tension of the winding line grips the ropes in the event of a drop in tension in the winding line. Devices such as those disclosed in U.S. Pat. Nos. 2,483,828 and 2,747,690 use clutches or brake rods to grip the wall of a shaft. Such devices work well for the type of lifting mechanisms for which they are designed, but are needlessly expensive and complex for many applications. For example, in a scissors mechanism of an agricultural grain auger and in devices for lifting small, inexpensive, or lightweight payloads, particularly those lifted without a vertical shaft, such devices would be commercially impractical.
The object of the present invention, therefore, is to provide an inexpensive, easy to manufacture and reliable safety device for cooperation with an active lifting mechanism, which device will secure the payload and mechanism in its raised position or slow its descent in the event of a failure of the primary lifting system, but which safety device sustains minimal loads and thus a minimum of wear during normal operation.